1. Field of the Invention
The present invention relates to a bolted connector for steel frame structures and more particularly, to a die cast bolted connector for use in steel frame structures to effect a bolted connection between a beam and a column.
2. Description of the Related Art
Steel frame structures such as buildings and the like are typically constructed either with welded connections or bolted fittings between beams and columns to achieve an assembly capable of bracing structures against lateral loads, such as earthquakes, wind, etc.
While welded connections are typically satisfactory under most normal loading conditions, these connections may be inadequate when the structure is subjected to abnormal loading conditions such as those that occur during earthquakes or other catastrophic events. The high loading forces experienced by structures during earthquakes are often sufficient to fracture or otherwise damage a welded connection. Thus, the welded connections tend to be the point of structural failure in welded steel frame structures.
Both the known welded and known bolted connections attempt to protect the columns and joints while promoting failure in the beams rather than at the joints. This practice has resulted in solutions which reduce the economy of the steel frame system. The known connections promote hinging/failure in the beam several feet away from the center of the column. Because this hinging occurs several feet from the center of the column, it magnifies the forces experienced by the column (from secondary moments/forces) thus increasing the size columns needed and thereby decreasing the economy of the structural steel frame assembly.
The column size may typically need to be further increased over and a earthquake force requirements to maintain superior strength over the beam which must remain the weak link. Because the structural steel yield and ultimate strengths of the beams can vary greatly due to structural steel milling practices and availability of raw material, maintaining column superior strength over the beams must be realized by using the worst case upper limit of the strength of the beams. For example, 50 ksi minimum yield steel may actually be 65 ksi yield. This increases column sizes which further reduces the economy of these known structures.
In addition, because the known structures promote failure of the beam, the cost of repairing the beams becomes a considerable expense due to the size of the beams, the lack of adequate access to the beams once the structure has been enclosed by finishes, and the need to weld near combustible materials. This reduces further the economy of the systems which promote hinging/failure of the beam.
An alternative to welded connections is a semi-rigid connection employing a bolted fitting. One type of bolted fitting for connecting columns and beams which has been used in place of a welded connection employs a T-shaped fitting to provide a bolted connection between the beam and the column. This type of T-shaped connection includes a base plate (top of T) which is bolted to the column and a protrusion which is bolted to the flange of an I-beam by high strength bolts. In connecting a beam to a column, two T-shaped fittings are generally attached by the protrusion to the top and bottom flanges of the I-beam. The base plate of the T-shaped fitting is attached by bolts to the column. However, the bolts which connect the base plate of the fitting to the column may tend to experience a lever effect when the flange of the beam to which the protrusion is bolted is subjected to a tensile force. This lever effect or prying action places a high stress on the bolts. Therefore, various attempts have been made to strengthen these bolted connections. The fittings themselves have been strengthened to such a degree that structures employing these types of fittings tend to have failures which occur in the beams at the location where the connectors end several feet from the center of the column.
T-shaped fittings are illustrated in U.S. Pat. Nos. 3,960,458, 3,938,297, 4,014,089, and 4,074,947.